Optical Spectroscopy of Waveguide coupled Er${}^{3+}$ ensembles in CaWO${}_4$ and YVO${}_4$

Abstract

We present an optical study of near-surface Er${}^{3+}$ ensembles in waveguide-integrated CaWO${}_4$ and YVO${}_4$, investigating how nanophotonic coupling modifies rare-earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In Er${}^{3+}$:CaWO${}_4$, we observe a pronounced polarization-dependent surface effect. TE-coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low-energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature-dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. Er${}^{3+}$:YVO${}_4$ remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non-charge-neutral rare-earth systems are more susceptible to surface-induced decoherence sources than charge-neutral hosts.